Harvesting heads of multi-function machines currently employed in timber harvesting have a crosscut saw for felling and crosscutting timber. The crosscut saw conventionally comprises a so-called chain saw provided with a chain of cutters revolving round a guide bar. In felling, a tree trunk is gripped by a harvesting head, whereafter the trunk is cut by applying the crosscut saw against it. The cutter chain of the crosscut saw is generally driven by a power device, such as a hydraulic motor, activated by pressure medium. As the crosscut saw is applied against the trunk, the guide bar with its cutter chain is turned against the trunk also by means of pressure medium, for instance, by a hydraulic cylinder. Thereafter, the felled trunk is guided against delimbing means by feeding means of the harvesting head and the delimbed tree trunk is cut to logs of desired length.
In connection with crosscutting, more and more attention is currently paid to cutting damage caused in connection with sawing so as to reduce economic losses resulting therefrom. These damages have been widely researched, for instance, by a Swedish research institute, Trätek.
To prevent damage, the feed rate of crosscut saws in harvesting heads is conventionally set to be constant. However, the feed rate must then be set so low that the crosscut saw is able to saw also trees having high sawing resistance, for instance, thick or icy trees.
Because the constant feed rate does not provide the most optimal sawing result, currently are also known crosscut saw control systems, in which a feed pressure of a pressure medium applied to a crosscut saw motor is measured for defining the sawing resistance. One of these control systems suggests to adjust the feed rate of the saw by throttling stepwise the volume flow of the pressure medium applied to the cylinder. On the other hand, arrangements are also known, in which the feed rate of the saw is adjusted by measuring the feed pressure of the pressure medium applied to the motor so as to keep the feed pressure of the crosscut saw substantially constant.
Control systems are also known, in which a crosscut saw rotating rate is measured, and the aim is to keep the speed of the cutter chain substantially constant by adjusting the feed force of the saw as the sawing resistance varies.
In all the above-mentioned inventions, the aim is to speed up the sawing by measuring either the feed pressure of the pressure medium applied to the crosscut saw motor or the rotating rate of the cutter chain. The aim of these methods is to keep the loading of the crosscut saw constant. However, previous solutions have not been able to provide any considerable improvement in the speed of the sawing event, because keeping the loading constant does not necessarily lead to an optimal result in the sawing rate.